1. Field of the Invention
The present invention relates to a tensioner that applies appropriate tension to a running chain, belt, or the like.
2. Description of the Related Art
It has been common practice to use a tensioner for maintaining appropriate tension in a chain or the like. For example, a known chain guide mechanism, which slidably guides a transmission chain, such as an endless roller chain passing over respective sprockets of a crankshaft and of a cam shaft inside an engine room, uses a tensioner to bias a tensioner lever in order to maintain appropriate tension of the chain or the like.
The known tensioner includes a housing having a plunger bore open toward a front side, a plunger slidably inserted in the plunger bore, and a coil spring that biases the plunger toward the front side. In such a tensioner, oil is supplied to an oil pressure chamber formed between the plunger bore and the plunger, so that the plunger is biased toward the front side by the oil in the oil pressure chamber. As the plunger reciprocates, the oil flows through the small gap between the plunger and the plunger bore and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger.
It has been known to provide a relief mechanism for discharging the oil inside, the oil pressure chamber to the outside of the plunger when the oil pressure in the oil pressure chamber builds up, so as to maintain the oil pressure of the oil pressure chamber at an appropriate level. In one form of the relief mechanism, as shown in FIG. 8, it is known to provide a relief hole 122 in the plunger 120 to connect the inside and outside of the plunger 120, and to dispose an orifice member 160 inside the plunger 120.
A helical groove 161b is formed on an outer circumferential surface of the orifice member 160. The orifice member 160 is press-fitted into a plunger hole 123, and the gap between the inner circumferential surface of the plunger hole 123 and the groove 161b is used to adjust the amount of oil to be discharged from the relief hole 122.
In this example shown in FIG. 8, however, since the orifice member 160 is installed inside the plunger hole 123 by press-fitting, the length in the front to back direction (axial direction) of the orifice member 160 needs to be set large so as to secure a press-fitting allowance for the orifice member 160, and the outer diameter of the orifice member 160 and the inner diameter of the plunger hole 123 must be formed with high precision. Moreover, because of the possibility of deformation of the plunger 120 caused by the press-fitting or the orifice member 160, a polishing process is necessary after the assembly. Furthermore, the press-fitting process causes various other problems such as the need for providing press-fitting equipment.
As one possible solution to the problems described above, it has been proposed to provide a disc-like orifice member 160 with an oil groove (not shown) in a first main surface 161 thereof facing a plunger bottom 121 side, such as to be fixedly pressed against the plunger bottom 121 by a coil spring 140 as shown in FIG. 9 (see, for example, Japanese Patent Application Laid-open No. 2003-194164).
In the tensioner of Japanese Patent Application Laid-open No. 2003-194164, when the oil pressure in the oil pressure chamber 111 rises, the oil O inside the oil pressure chamber 111 is discharged to the outside of the plunger 120, traveling through the gap between the outer circumferential surface of the orifice member 160 and the inner circumferential surface of the plunger hole 123, the oil groove in the first main surface 161, and the relief hole 122.
This relief mechanism described in Japanese Patent Application Laid-open No. 2003-194164 also serves as a degassing mechanism for expelling the air A that has entered the oil pressure chamber 111 to the outside of the oil pressure chamber 111. More specifically, the air A that has entered the oil pressure chamber 111 is discharged to the outside of the plunger 120, traveling through the gap between the outer circumferential surface of the orifice member 160 and the inner circumferential surface of the plunger hole 123, the oil groove in the first main surface 161, and the relief hole 122, similarly to the oil O.